R2Fe17 compounds wherein R is selected from rare earth elements inclusive of yttrium are intermetallic compounds having either a Th2Zn17 type rhombohedral crystal structure or a Th2Ni17 type hexagonal crystal structure. While permanent magnet materials must meet the three major requirements: (a) high saturation magnetization, (b) a high Curie temperature, and (c) a high crystal magnetic anisotropy constant, these compounds, which satisfy only requirement (a), have not been considered as a candidate for permanent magnets. However, around 1990, Coey et al. and Iriyama et al. discovered that interstitial incorporation of nitrogen (N) into R2Fe17 compounds drastically alters their magnetic properties. See J. M. D. Coey and H. Sun, Journal of Magnetism and Magnetic Materials, 87 (1990), L 251; H. Imai and T. Iriyama, Japanese Application No. 228547/88, 1988; T. Irlyama, K. Kobayashi and H. Imai EP 0369097 A1, 1989. It is possible to incorporate at most three N atoms per compositional formula: R2Fe17Nx and at sites surrounding R atoms. As a result of N atoms incorporated, the lattice constant is elongated in both a and c axes, leading to a lattice expansion of at least several percents by volume. For all compounds having N incorporated therein, substantial increases of Curie temperature (Tc) are found. Crystal magnetic anisotropy changes from a negative value prior to nitriding to a large positive value of the order of 107 erg/cm3 in the case of Sm2Fe17N3. In the cases of Nd and Pr systems, their crystal magnetic anisotropy remains negative because the orbit of 4f electrons in rare earth atom responsible for magnetism is flattened (as opposed to the cigar shape of the Sm system). The Sm2Fe17N3 compound has a saturation magnetization of 15.6 kG which is comparable to that (16 kG) of NdFeB compounds. Therefore, among R2Fe17N3 compounds, only Sm2Fe17N3 satisfies the three major requirements of permanent magnets and has a potential to become an excellent permanent magnet.
Nitriding of R2Fe17 is generally carried out by heating magnetic powder to a temperature below the decomposition temperature and placing the powder in a N2 gas atmosphere at the temperature. To this end, not only the N2 gas, but also a gas mixture of N2+H2 or a gas mixture of NH3+H2 may be used. These gas mixtures are advantageous in that magnetic particles are fully nitrided because H2 gas is occluded by the compound to bring about interstitial expansion whereby microcracks are induced in magnetic particles to accelerate diffusion of N2 or NH3 gas into magnetic particle surfaces. Sometimes N2 gas under high pressure is used.
R2Fe17N3 suffers from the problem that the nitride decomposes at about 600° C. or higher into RNx and Fe as shown by the following scheme. 
FIG. 1 is a diagram showing differential thermal analysis (DTA) curves of Sm2Fe17N3 magnetic powder when heated at different temperatures in an Ar gas atmosphere. It is seen that decomposition starts little by little from a temperature of 500° C. or above. Attempts were made to add an additive to the alloy to elevate the decomposition temperature, and marked a mere elevation within 100° C. at maximum. Since the sintering temperature used in the sintering of rare earth-transition metal compounds by powder metallurgy is usually at or above 1,100° C., it is difficult to work the nitride powder into a bulk magnet by powder metallurgy. It may be devised to subject the sintered body to nitriding, although it is difficult to effect nitriding throughout the body in the bulk compound state because nitriding takes place through surface diffusion. Therefore, no reports showing a success in producing Sm2Fe17N3 magnet in bulk form have been found in the art except for the pulse ultrahigh pressure process using a gas gun. The pulse ultrahigh pressure process involves charging the target of the gas gun with a magnetic powder and striking the target against a barrier to apply instantaneous pulse impact pressures and is utterly unacceptable in the industry.
For the above reason, the R2Fe17N3 magnetic powder composed mainly of Sm2Fe17N3 is used to produce bonded magnets because the powder can be processed as such. Since Sm2Fe17N3 has a significant anisotropic magnetic field, a practically satisfactory coercivity is obtained in fine particle form. By placing the fine particles in a magnetic field for orientation, an anisotropic bonded magnet can be produced. (BH)max values of approximately 20 MGOe (160 kJ/m3) have been reported, though on the laboratory level.
Although the R2Fe17N3 magnet composed mainly of Sm2Fe17N3 exhibits more or less satisfactory characteristics in anisotropic bonded magnet form, its application is limited because it cannot be converted into a bulk body by a practically acceptable method.